The present invention relates to an electrometer for detecting a surface potential of an object to be measured and, particularly, to an electrometer having a function of converting a surface potential into an A.C. signal by means of an electromechanical resonator.
A conventional electrometer includes a casing having a measuring hole for introducing electric line of force emitted from an object to be measured. A tuning fork is provided with a chopper portion for chopping the electric line of force introduced in the casing with a constant period. A detection electrode is responsive to the chopped electric line of force to derive an A.C. signal therefrom. Such electrometer, however, is bulky due to its large width size, for reasons to be described later. Further, an output of such electrometer varies according to a mutual positional relation between the measuring hole, the chopper portion and the detection electrode. In order to solve these problems, an electrometer has been proposed in which an orientation of a tuning fork is changed. In such electrometer having the tuning fork the orientation of which is changed, however, physical characteristics, particularly, stiffness thereof, of an adhesive used to fix a lead wire for leading an output signal thereof onto a surface of an arm of the tuning fork is very sensitive to temperature compared with metal material forming the tuning fork. Therefore, a vibration amplitude of the tuning fork varies considerably with temperature, so that an operation of the electrometer becomes unstable with respect to temperature.